There is no constitutional law of criminal law per se, yet there are a number of constitutional boundaries dividing what legislatures can criminalize and what they cannot. This short comment, prompted by Jack Chin's new revisionist history of Yick Wo v. Hopkins, describes what Yick Wo represents about the constitutional regulation of substantive criminal law. The Supreme Court's protection of property rights under Lochner-era due process doctrine demonstrates one of the doctrines by which the Court indirectly regulated legislatures' substantive crime definition. Due process review of property and contract interests provided the first significant constraint on legislative definition of substantive criminal law which, roughly until the time of Yick Wo, was almost completely unconstrained by courts. The Court employed substantive due process doctrine in a variety of ways to strike down legislative crime definition, such as distinguishing between livelihoods that were not inherently lawful callings and those like Yick Wo's laundry, which was a business harmless in itself and useful to the community and thereby in a protected sphere less subject to criminalization or other regulation. But parameters were always drawn incidentally to the regulation's criminal form; criminal law's reach is, as a matter of constitutional law, co-extensive with legislatures' general regulatory authority. And even with the demise of Lochner-era jurisprudence and the rise of a range of other constitutional doctrines, that has been the case in the 120 years since Yick Wo.